Because of its low density as well as excellent corrosion resistance, high-temperature resistance, high-temperature oxidation resistance and thermal shock resistance, porous SiC ceramic material can be widely used in molten metal filters, catalyst carriers, water purifiers, thermal insulation and sound absorbing materials, gas combustion chamber media and high-temperature gas filters.
The preparation of high-strength porous SiC ceramic material generally requires a very high sintering temperature, usually above 1,800° C. The production process of such material can cause high energy consumption. Reducing the sintering temperature has been a research focus. Currently, there are two widely used low-temperature sintering technologies: one is to add inorganic metal oxides to react with SiC aggregate to form a new phase to strengthen the neck link of the particles, thereby enhancing the strength of SiC ceramic material; the other is to use polycarbonate silane as the precursor to prepare porous SiC ceramics at lower temperatures, but this method is costly and is difficult to produce and use on a large scale. Zircon is a chemically inert substance and has a strong corrosion resistance, and it is also difficult to react with molten metal and has a high stability in the high-temperature exhaust gas. Because zircon and SiC have very close thermal expansion coefficient, they can be well combined and have excellent high-temperature thermal shock resistance and is one of high-temperature dustproof ceramic materials with broad application prospects. However, there are many problems in the preparation of zircon-SiC porous ceramics. For example, porous ceramics prepared with SiC having a small particle size and the pore-forming agent have insufficient pore connectivity, small pore size and other defects, resulting in low gas permeability. This can't meet the high gas permeability required for high-temperature gas filtration and dust removal. Zircon is usually formed by letting zirconia and SiC react in the aerobic atmosphere. If SiC aggregate having a size of more than 100 microns is used to make zircon ceramics, a problem will also occur, that is, only a small part of the surface of SiC particles can be exposed to air, so the SiC particles can't be completely oxidized and a little zircon can form only in the neck surface of the particles. The porous ceramic prepared this way has very small strength and can't meet the needs of industrial production and can't be used as a high-temperature gas dust removal material. SiC whiskers have excellent mechanical properties, heat resistance, corrosion resistance and high-temperature oxidation resistance and have become one of the main reinforcing agents for a variety of high-performance composite materials.
For example, the Chinese patent application file (Publication No. CN102161594A) discloses a SiC whiskers-reinforced SiC ceramic matrix composite material, whose highest flexural strength was found to reach up to 500 MPa. The composite material has excellent comprehensive properties. Another example is that the Chinese patent application file (Publication No. CN100545127C) discloses a SiC whiskers-toughened titanium carbonitride-based cermet cutting blade which has high strength, high toughness and better abrasion resistance and heat resistance. However, the above patents are all related to the field of functional ceramics, and there are few studies on the use of SiC whiskers to reinforce the porous ceramic materials. In particular, there have not yet been published studies about the use of SiC whiskers for the preparation of SiC porous ceramic materials.
For porous SiC ceramic materials used for high-temperature flue gas purification, the flexural strength, porosity and average pore size are very important indicators, because the strength can withstand the impact of airflow, thus extending the service life of the materials; porosity and the average pore size mean a higher gas flux. In the existing technologies, large particles of SiC are difficult to oxidize, resulting in a small amount of zircon produced, so the flexural strength of the resulted ceramic material is not high, making it difficult to realize large-scale applications. Therefore, it is of great significance to use SiC whiskers to reinforce porous SiC materials to meet the requirements of high-temperature flue gas purification.
The purpose of the invention is to improve the strength and porosity of SiC porous ceramics at the same time to obtain a type of SiC porous ceramic material for high-temperature filtration.
The inventors of the invention have found that the addition of certain SiC whiskers into the mixture using SiC as aggregate can significantly improve the flexural strength of SiC porous ceramics, and the finally obtained SiC porous ceramics still have high porosity and good heat shock resistance and can meet the application requirements.
In order to achieve the above objective, the technical scheme used by the invention is:
A method for preparing a type of whisker-reinforced SiC porous ceramic material was proposed. Raw materials for preparing such material include large-particle SiC aggregate, a sintering aid, a pore-forming agent, and a type of macromolecular polymer for bonding, and another type of macromolecular polymer used for reducing the mold-release resistance. The feature of the method lies in that the blending materials also include a type of reinforcing agent, that is, SiC whiskers.
The method described in the invention is characterized by that the said sintering aid is zirconia, the said pore-forming agent is activated carbon, the said macromolecular polymer for bonding is polyvinyl alcohol (PVA), and the said macromolecular polymer used for reducing the mold-release resistance is liquid paraffin.
The method described in the invention is characterized by that the steps of preparing SiC porous ceramic material are:    (1) SiC aggregate, zirconia, activated carbon and SiC whiskers are mixed and then mechanically ground via ball milling to obtain a mixed powder a;    (2) Add PVA and liquid paraffin into the mixed powder a and mix them evenly, and then make the mixture into SiC porous ceramic green body through the dry pressing or extrusion method, and then dry the green body in the oven to get green body b;    (3) Green body b is placed in a high-temperature furnace to sinter. After natural cooling, a type of porous ceramic material can be obtained.
The method described in the invention is characterized by that the milling time in step (1) is 2-10 h until powder a is mixed evenly.
The method described in the invention is characterized by that the drying temperature of step (2) is 60 to 90° C. and the drying time is 2 to 4 hours.
The method described in the invention is characterized by that the sintering process mentioned in step (3) uses the following steps: first raise the temperature to 200° C. at a rate of 1-2° C./min; then raise the temperature to 500° C. at a rate of 2-3° C./min; keep the temperature at 500° C. for 0.5˜1 h to remove the activated carbon; finally raise the temperature to 1,450˜1,650° C. at a rate of 3˜3.5° C./min and keep the temperature at the point for 2˜6 h.
The method described in the invention is characterized by that the shapes of the said green bodies include plate, tube and honeycomb.
The method described in the invention is characterized by that the said SiC aggregate has an average particle size of 20-300 μm; the said ZrO2 has an average particle size of 1-5 μm and a purity of more than 99%; the said SiC whiskers have a crystal form of β and L/D>20, wherein L is a length of a SiC whisker and D is a diameter of the SiC whisker; the said activated carbon has a particle size of 8-50 μm; the said PVA is 1-10% PVA aqueous solution; and the ratio (mass ratio) of the said mixture is: zirconia occupies 5-20%, SiC whiskers 1-10%, activated carbon 5-15%, PVA 5%, and the remaining is SiC aggregate.
The method described in the invention is characterized by that a type of porous ceramic membrane material is obtained according to the method.
The method described in the invention is characterized by that the resulted porous ceramic membrane material is applied in the high-temperature flue gas dust removal and vehicle exhaust purification. The principle of the method described in the invention is as follows: the porous structure is obtained by the accumulation of particles and the addition of the pore-forming agent; the sintering aid is added into the mixture to enhance the bonding between the particles by means of the reactive sintering technology; for the problem that large SiC particles are difficult to oxidize at high temperature and this results in the too low strength of the prepared ceramic sample, SiC whiskers are added into the mixture to react with ZrO2 to produce zircon to reinforce the neck connection among SiC particles, thereby enhancing the ceramic strength.
The main content of the invention is to prepare a type of whisker-reinforced SiC porous ceramics, a type of zircon-SiC porous ceramics having a high flexural strength, and to use it in high-temperature flue gas filtration, especially in large thermal power plants, steel plants and other industries where coal is massively consumed.
In the invention, various parameters are determined with the following methods:    1. The porosity is tested with the Archimedes principle, with deionized water as the soaking medium. For the specific steps, please refer to relevant literature (reference: Physical Testing and Chemical Analysis-Part A: Physical Testing: Methods for determining the density and porosity of ceramic materials. (2006): 42(6): 289-291).    2. The flexural strength is tested with the flexural strength tester to measure the average flexural strength value of three points, with a span of 40 mm and a loading speed of 0.5 mm/min. As for the specific test steps, please refer to the national standard GB/T1965-1996 (reference: the national standard of the People's Republic of China: Test method for the flexural strength of porous ceramics (GB/T1965-1996), released on Sep. 13, 1996 and implemented on Apr. 1, 1997).    3. The pore size is tested with the gas bubble pressure method, with deionized water as the wetting agent. As for the specific test steps, refer to the relevant literature (reference: Inorganic Membrane Separation Technology and Application, by Xu Nanping, Xing Weihong and Zhao Yijiang, Chemical Industry Press, 2003, 21-22).    4. The test method of the gas permeation performance is to test the gas flow at the infiltration side under different pressures, using a gas-solid separation device made by the lab (reference: Preparation and Characterization of Porous Materials, by Chen Yong, China University of Science and Technology Press, 2010, 249-250).
In comparison with the existing preparation techniques, the zircon-SiC porous ceramics prepared in the invention has the following positive effects:    (1) Improve the strength of porous ceramics to ensure its application in the actual process.    (2) Provide a method for the preparation of high-strength porous ceramics, and provide an idea for the preparation of other porous ceramic materials.    (3) Prepare high-porosity new-type SiC porous ceramic materials, improve their gas permeability and expand their practical applications.